Clearer error message if pyelftools is not installed

mojo/devtools/common/android_gdb/session.py

 # Copyright 2015 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """
 Manages a debugging session with GDB.
 
 This module is meant to be imported from inside GDB. Once loaded, the
 |DebugSession| attaches GDB to a running Mojo Shell process on an Android
 device using a remote gdbserver.
@@ skipping 69 matching lines @@
     self._package_name = package_name
     self._adb = adb
     self._remote_file_cache = os.path.join(os.getenv('HOME'), '.mojosymbols')
 
     if pyelftools_dir != None:
       sys.path.append(pyelftools_dir)
     try:
       import elftools.elf.elffile as elffile
     except ImportError:
       logging.fatal("Unable to find elftools module; please install it "
-                    "(for exmple, using 'pip install elftools')")
+                    "(for example, using 'pip install pyelftools')")
       sys.exit(1)
 
     self._elffile_module = elffile
 
     self._libraries = self._find_libraries(build_directory)
     self._rfc = RemoteFileConnection('localhost', 10000)
     self._remote_file_reader_process = None
     if not os.path.exists(self._remote_file_cache):
       os.makedirs(self._remote_file_cache)
     self._done_mapping = set()
@@ skipping 163 matching lines @@
     subprocess.check_call([self._adb, 'forward', 'tcp:10000', 'tcp:%d' % port])
     self._rfc.connect()
 
     _gdb_execute('target remote localhost:9999')
 
     self._update_symbols()
     def on_stop(_):
       self._update_symbols()
     gdb.events.stop.connect(on_stop)
     gdb.events.exited.connect(self.stop)